<!DOCTYPE html>
<html>
<head>
    <title>Clustered Features</title>
    <link rel="stylesheet" href="https://openlayers.org/en/v4.6.5/css/ol.css" type="text/css">
    <!-- The line below is only needed for old environments like Internet Explorer and Android 4.x -->
    <script src="https://cdn.polyfill.io/v2/polyfill.min.js?features=requestAnimationFrame,Element.prototype.classList,URL"></script>
    <script src="../js/ol-debug.js"></script>
<!--    <script src="https://openlayers.org/en/v4.6.5/build/ol.js"></script>-->
</head>
<body>
<div id="map" class="map"></div>
<form>
    <label>cluster distance</label>
    <input id="distance" type="range" min="0" max="100" step="1" value="40"/>
</form>
<script>
    var distance = document.getElementById('distance');

    var count = 20000;
    var features = new Array(count);
    var e = 4500000;
    for (var i = 0; i < count; ++i) {
        var coordinates = [2 * e * Math.random() - e, 2 * e * Math.random() - e];
        features[i] = new ol.Feature(new ol.geom.Point(coordinates));
    }

    var source = new ol.source.Vector({
        features: features
    });

    var clusterSource = new ol.source.Cluster({
        distance: parseInt(distance.value, 10),
        source: source
    });

    var styleCache = {};
    var clusters = new ol.layer.Vector({
        source: clusterSource,
        style: function(feature) {
            console.log('%c clusterSource: ','color: MidnightBlue; background: Aquamarine; font-size: 20px;', clusterSource.distance);

            console.log('%c feature: ','color: MidnightBlue; background: Aquamarine; font-size: 20px;', feature);

            var size = feature.get('features').length;
            var style = styleCache[size];
            if (!style) {
                style = new ol.style.Style({
                    image: new ol.style.Circle({
                        radius: 10,
                        stroke: new ol.style.Stroke({
                            color: '#fff'
                        }),
                        fill: new ol.style.Fill({
                            color: '#3399CC'
                        })
                    }),
                    text: new ol.style.Text({
                        text: size.toString(),
                        fill: new ol.style.Fill({
                            color: '#fff'
                        })
                    })
                });
                styleCache[size] = style;
            }
            return style;
        }
    });

    var raster = new ol.layer.Tile({
        source: new ol.source.OSM(),

    });
    // raster.setExtent([-180, -90, 180, 90])


    var map = new ol.Map({
        layers: [raster, clusters],
        target: 'map',
        view: new ol.View({
            center: [0, 0],
            zoom: 2
        })
    });

    distance.addEventListener('input', function() {
        clusterSource.setDistance(parseInt(distance.value, 10));
    });

</script>
</body>
</html>